UDC networks support a layered architecture that separates user data from application logic. In a UDC network, the user data are stored centrally in a logically unique entity called User Data Repository (UDR). An exemplary realisation of a UDC network with a UDR is described in 3rd Generation Partnership Project (3GPP) Technical Specification (TS) 23.335 V.9.0.0 (2010-03).
In TS 23.335, the entities that need to access user data stored in the UDR are collectively referred to as application Front Ends (FEs). The application FEs keep the application logic for handling the user data retrieved from the UDR and connect to the UDR through a so-called Ud reference point. Examples of application FEs include the HLR, Home Subscriber Server (HSS), Access Network Discovery and Selection Function in Home Network (H-ANDSF), Application Servers (ASs), and so on.
Compared to a non-UDC network, in a UDC network the capacity of an individual HLR (i.e., of an HLR-FE) is drastically increased. The capacity increase results from the fact that the HLR-FE can be realized as a dataless entity (as the subscription data are stored centrally in the UDR, see FIG. 1). An HLR-FE will thus be in charge of handling a significantly larger number of subscriptions than a conventional HLR in a non-UDC network. However, the capacity increase also has certain drawbacks, and such draw-backs exist in particular in case of an outage of, for example, the UDR.
In case of an outage of a conventional HLR, the recovery process is defined in 3GPP TS 23.007. Specifically, the HLR is to send a reset message to each Visitor Location Register (VLR) in which one of its subscribers is registered. This message causes each concerned VLR to mark the relevant subscriber record by “Location Information Not Confirmed in HLR”. The marking triggers, upon an activity of the related subscriber, a location update procedure towards the HLR so as to update VLR subscription information (via Insert Subscriber Data messages) and the VLR address stored by the HLR. A similar recovery process is defined for the interface between the HLR and an associated Serving GPRS Support Node (SGSN).
As will be appreciated, the reset message sent by the HLR after an outage triggers a burst of Location Update messages towards the HLR (and a subsequent burst of Insert Subscriber Data messages to the VLR/SGSN). While in conventional networks such message bursts could be handled, the HLR capacity increase in UDC networks is expected to result in unacceptably high peak loads for HLR-FEs and the UDR. To address this drawback the reset messages could be sent one-by-one by the HLR-FE, but then the recovery time would increase to an extent that the nightly maintenance window may not be long enough to facilitate the corrective Location Update procedures.